Method and apparatus to limit output power in a switching power supply

ABSTRACT

Techniques are disclosed to adjust a current limit in a switching regulator. One example switching regulator includes a switch to be coupled to an energy transfer element of a power supply. A controller to generate a drive signal is coupled to control switching of the switch to regulate an output of the power supply. A current limiter is included in the controller to adjust the drive signal to limit a current though the switch to a variable current limit value. The variable current limit value is set by the current limiter in response to an input line voltage to be coupled to the energy transfer element.

REFERENCE TO PRIOR APPLICATIONS

This application is a continuation of U.S. application Ser. No.11/179,144, filed Jul. 11, 2005, now U.S. Pat. No. 7,215,107, entitled“Method and Apparatus to Limit Output Power in a Switching PowerSupply.”

BACKGROUND

1. Technical Field

The present invention relates generally to electronic circuits, and morespecifically, the invention relates to switched mode power supplies.

2. Background Information

A typical requirement for power supplies of electronic equipment is thatthey limit their output power. One reason to limit output power is tomeet the requirements of safety agencies for prevention of personalinjury. Another reason to limit output power is to avoid damage toelectronic components from an overload.

Power supplies typically have self-protection circuits that respond whenan output becomes unregulated for a specified time. However, if outputpower is not limited, a fault at a load can consume enough power tocause damage or to exceed regulatory requirements while the outputsremain regulated. Thus, the self-protection feature can be ineffectiveif the power supply can deliver too much power.

A common way to limit output power of a switching power supply is tolimit the current in a power switch at the input of the power supply.The maximum output power is related to the peak current in the switch.Inherent delays in the responses of electrical circuits create an errorbetween the desired limit for peak current in the switch and the actualmaximum peak current in the switch. The error is greater at higher inputvoltages, causing the maximum output power to be greater at higher inputvoltages than it is at lower input voltages.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention detailed illustrated by way of example and notlimitation in the accompanying Figures.

FIG. 1 is a functional block diagram of one embodiment of a switchingpower supply that may limit output power in accordance with the teachingof the present invention.

FIG. 2 is a graph of power capability for one embodiment of a switchingpower supply with respect to the peak current of the switch.

FIG. 3 shows waveforms of the current in the switch for one embodimentof a switching power supply in accordance with the teaching of thepresent invention.

FIG. 4 shows parameters of timing signals with parameters of the currentin a switch of a power supply that may limit output power in accordancewith the teaching of the present invention.

FIG. 5 is a flow diagram that illustrates a method to limit output powerof a switching power supply in accordance with the teaching of thepresent invention.

FIG. 6 shows timing signals with waveforms of the current in a switch ofa switching power supply to illustrate operation of one embodiment ofthe present invention.

FIG. 7 is a functional block diagram of one embodiment of the presentinvention that includes the power switch in an integrated circuit.

DETAILED DESCRIPTION

Embodiments of a power supply regulator that may be utilized in a powersupply are disclosed. In the following description, numerous specificdetails are set forth in order to provide a thorough understanding ofthe present invention. It will be apparent, however, to one havingordinary skill in the art that the specific detail need not be employedto practice the present invention. Well-known methods related to theimplementation have not been described in detail in order to avoidobscuring the present invention.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present invention. Thus, theappearances of the phrases “for one embodiment” or “in an embodiment” invarious places throughout this specification are not necessarily allreferring to the same embodiment. Furthermore, the particular features,structures, or characteristics may be combined in any suitable manner inone or more embodiments.

As will be discussed, the power from a switching power supply may belimited according to embodiments of the present invention by limitingthe current in a switch of the power supply. For one embodiment, aswitch is coupled to an energy transfer element of a power supply with acontroller generating a drive signal to control switching of the switchto regulate the output of the power supply. The controller includes acurrent limiter, which will adjust the drive signal to limit a currentthough the switch to a variable current limit value. For one embodiment,the current limiter based on the input line voltage of the power supplysets the variable current limit value. For example, the variable currentlimit may be to a nominal current limit value for nominal or a low inputline voltage. If, however, the input line voltage is relatively high,then the variable current limit is set to a reduced current limit valuein accordance with the teachings of the present invention. For oneembodiment, the controller deduces the magnitude of the input voltage bymeasuring how long the current takes to go between two values and thevariable current limit is then adjusted accordingly.

The variable current limit value for the switch is adjusted according tothe input voltage of the power supply to compensate for a delay betweenthe time when the current reaches the current limit and the time whenthe switch turns off. A lower value of current limit for higher inputvoltages prevents excess output power at high input voltage. For oneembodiment, input voltage may be determined indirectly from ameasurement of time to reach current limit from an initial value of zerocurrent when the power supply operates in discontinuous conduction mode.In general, a design can deliver a required output power and also limitthe maximum output power over the operating range of input voltage bycompensating for the error between desired maximum peak current in theswitch and the actual maximum peak current in the switch.

As will be discussed, a measurement of time is used to determine anappropriate adjustment of the desired maximum peak current in the switchto meet the requirements of the design. A current limit threshold for aswitch is adjusted in response to a measurement of time during theconduction of the switch to compensate for the undesirable influence ofinput line voltage on the actual peak current in the switch. Forinstance, a relatively high input line voltage is indicated for anembodiment of the present invention if an over current condition isidentified during a first switching cycle after a skipped switchingcycle of the switch.

As will be discussed, it is likely that the power supply will operate ina discontinuous conduction mode of operation in the first switchingcycle after a skipped switching cycle. In this situation, the energy inthe energy transfer element typically goes to zero before the switchturns on in the next switching cycle. Therefore, if an over currentcondition occurs during this first switching cycle with the energy inthe energy transfer element initially at zero at the beginning of theswitching cycle, a high input line condition is indicated, and thevariable current limit is set accordingly to the reduced value inaccordance with the teachings of the present invention. If, on the otherhand, an over current condition is not identified in the first switchingcycle after a skipped switching cycle of the switch, then it is assumedthat the input line voltage of the power supply is nominal or relativelylow, and the variable current limit is set accordingly to the nominalvalue in accordance with the teachings of the present invention.

To illustrate, FIG. 1 shows a functional block diagram of a power supplythat may include an embodiment of a method that limits peak switchcurrent in accordance with the teachings of the present invention. Thetopology of the power supply illustrated in FIG. 1 is known as a flybackregulator. It is appreciated that there are many topologies andconfigurations of switching regulators, and that the flyback topologyshown in FIG. 1 is provided to illustrate the principles of anembodiment of the present invention that may apply also to other typesof topologies in accordance with the teachings of the present invention.

As illustrated in the power supply example of FIG. 1, an energy transferelement T1 125 is coupled between an unregulated input voltage V_(IN)105 and a load 165 at an output of the power supply. A switch S1 120 iscoupled to the primary winding 175 at an input of energy transferelement 125 to regulate the transfer of energy from the unregulatedinput voltage V_(IN) 105 to the load 165 at the output of the powersupply. A controller 145 is coupled to generate a drive signal 157 thatis coupled to be received by the switch Si 120 to control switching ofswitch S1 120. In the example of FIG. 1, the energy transfer element T1125 is illustrated as a transformer with two windings. A primary winding175 has N_(p) turns with an inductance L_(p). A secondary winding hasN_(S) turns. In general, the transformer can have more than twowindings, with additional windings to provide power to additional loads,to provide bias voltages, or to sense the voltage at a load.

A clamp circuit 110 is coupled to the primary winding 175 of the energytransfer element T1 125 to control the maximum voltage on the switch S1120. In one embodiment, switch S1 120 is a transistor such as forexample a power metal oxide semiconductor field effect transistor(MOSFET). In one embodiment, controller 145 includes integrated circuitsand discrete electrical components. The operation of switch S1 120produces pulsating current in the rectifier D1 130 that is filtered bycapacitor C1 135 to produce a substantially constant output voltageV_(O) or a substantially constant output current I_(O) at the load 165.

The output quantity to be regulated is U_(O) 150, that in general couldbe an output voltage V_(O), an output current I_(O), or a combination ofthe two. A feedback circuit 160 is coupled to the output quantity U_(O)150 to produce a feedback signal U_(FB) 155 that is an input to thecontroller 145. Controller 145 also includes a current sensor coupled toreceive current sense 140 that senses a current I_(D) 115 in switch S1120. Any of the many known ways to measure a switched current, such asfor example a current transformer, or for example the voltage across adiscrete resistor, or for example the voltage across a transistor whenthe transistor is conducting, may be used to measure current I_(D) 115.The controller may use current sense signal 140 to regulate the outputU_(O) 150 or to prevent damage to the switch S1 120.

FIG. 1 also shows an example waveform for current I_(D) 115. During anyswitching period T_(S) 190, switch S1 120 may conduct in response todrive signal 157 from controller 145 to regulate the output U_(O) 150.When current I_(D) 115 reaches a current limit value I_(PEAK) 195 aftera time t_(ON) 180 from the beginning of the switching period T_(S) 190,switch S1 120 turns off and stays off for a time t_(OFF) 185, which isthe remainder of the switching period T_(S) 190. The current waveformshows two fundamental modes of operation. The trapezoidal shape 170 ischaracteristic of continuous conduction mode (CCM) whereas thetriangular shape 175 is characteristic of discontinuous conduction mode(DCM).

FIG. 2 shows how the peak current I_(PEAK) 190 is related to the maximumoutput power of the power supply in FIG. 1. In DCM, the output powerincreases as the square of I_(PEAK). In CCM, the output power increaseslinearly with I_(PEAK). The current limit value I_(PEAK) is used to helplimit the output power of the power supply. A difficulty in limiting thecurrent limit value I_(PEAK) is that there is always a delay between thetime when the current reaches the limit and the time the switch turnsoff.

To illustrate, FIG. 3 shows how a delay influences peak current in theswitch. In the example illustrated in FIG. 3, I_(PMAX) is the maximumdesired value for I_(D). A controller having a current limit thresholdI_(LIMIT1) that is the same value of I_(PMAX) takes action to turn offthe switch when I_(D) exceeds I_(LIMIT1). The unavoidable time delayt_(d) allows I_(D) to exceed I_(LIMIT1) by an amount ΔI_(DELAY) thatdepends on the delay t_(d) and on how fast I_(D) is changing after itpasses I_(LIMIT1). A current limit I_(LIMIT1) produces a peak currentI_(PEAK1) that is greater than the desired I_(PMAX). If the delay t_(d)and the rate of change of I_(D) are known, the current limit can becompensated to a lower value I_(LIMIT2) such that addition of ΔI_(DELAY)will give a peak current I_(PEAK2) that is less than I_(PMAX).

A complication in the use of a lower current limit value to compensatefor the delay is that in general ΔI_(DELAY) will be larger at higherinput voltages than at lower input voltages because I_(D) increases at agreater rate when the input voltage is high. Therefore, a power supplythat uses a single compensated current limit I_(LIMIT2) to limit maximumoutput power to the desired value at a high input voltage would haveless than the desired maximum output power at low input voltage. Indeed,if the circuit to limit the power has only one desired limit for peakcurrent such as I_(PEAK2), a design that meets the requirement formaximum power at high input voltage may be unable to deliver therequired power at low input voltage.

For one embodiment, a power supply may use a first compensated currentlimit I_(LIMIT1) at a low input voltage and a second compensated currentlimit I_(LIMIT2) at high input voltage to limit the maximum output powerto a desired value over a wide range of input voltages in accordancewith the teachings of the present invention.

To illustrate, FIG. 4 shows example timing signals that are used withthe current I_(D) for one embodiment of the invention to determinewhether the current limit will be I_(LIMIT1) or I_(LIMIT2). Inparticular, FIG. 4 shows two full switching periods, T1 and T2 of switchcurrent I_(D) with timing signals I_(LIM), I_(LIMMAX), and D_(MAX). InFIG. 4, current limit signal I_(LIM) is high whenever I_(D) is greaterthan the current limit. Signal I_(LIMMAX) is a timing reference that iscompared to current signal I_(LIM) to determine whether the currentlimit will be I_(LIMIT1) or I_(LIMIT2). Signal D_(MAX) sets the maximumon time of the switch. The switch is forced off when D_(MAX) is high.

FIG. 5 is a flow diagram for one embodiment of a power supply controllerthat uses the timing signals of FIG. 4 in accordance with the teachingsof the present invention. The flow starts at Block 505 when the switchis off. Block 510 sets a nominal current limit, which for one embodimentcorresponds to I_(LIMIT1) in FIG. 4, and is more suitable for a nominalor low input voltage. Block 515 interprets feedback signal U_(FB) todetermine whether the switch should turn on or remain off in the nextswitching period. If the switch is enabled, then Block 520 directs theswitch to turn on in Block 525. If the switch is not enabled, then Block520 directs the switch to be off in Block 545.

Once the switch is turned on, the state of the current limit signalI_(LIM) is evaluated in Block 535. The on time of the switch is comparedto the maximum permissible on time in Block 540. Block 545 turns off theswitch immediately if I_(LIM) is high or if the on time exceeds themaximum on time t_(DMAX). After the switch turns off, Block 550 directsthe flow depending on whether the mode of operation was CCM or DCM whenthe switch turned on. The mode is DCM if the energy in the energytransfer element goes to zero before the switch turns on. In oneembodiment, a single switching period with the switch disabled issufficient to reduce the energy to zero. Therefore, in one embodiment,Block 550 has a memory of whether or not the switch was enabled during aprevious switching period to determine the mode of operation at thestart of the present switching period.

If the mode of operation was not DCM, the controller continues with theinterpretation of the feedback signal in Block 515. If the mode ofoperation was DCM, the flow is diverted to Block 555. Block 555 comparesthe time to reach current limit against the reference time t_(LIMMAX).Although delays in practical circuits prevent exact measurement of thetime t_(LIM) to reach the current limit, it is sufficient to measure asignal that includes the delays for an approximate measurement oft_(LIM). For one embodiment, the sum of t_(LIM) and delay t_(d), whichis the on time t_(ON) in FIG. 3, is measured in Block 555 as anapproximation to t_(LIM) for comparison against the reference timet_(LIMMAX). When the operation is in DCM, the current can reach currentlimit in less time than t_(LIMMAX) only if the input voltage is high inaccordance with the teachings of the present invention.

If the time to reach current limit is less than t_(LIMMAX), thecontroller sets a reduced current limit in Block 530. The reducedcurrent limit for a high input voltage corresponds to I_(LIMIT2) in FIG.4. If the time to reach current limit is not less than the referencetime t_(LIMMAX), then the controller sets the nominal current limit inBlock 510. The latter condition is also true when the switch turns offbefore the current reaches current limit, causing the controller to setthe nominal current limit in Block 510.

FIG. 6 shows several switching periods that illustrate operationaccording to the flow diagram of FIG. 5. In Period 1, the switchoperates at a high input voltage when the current limit has been set atthe nominal value I_(LIMIT1) that is appropriate for a nominal or lowinput voltage. The surplus energy from the high peak current at the highinput voltage causes the controller to disable the switch in Period 2.The controller detects a high input voltage condition from the shorttime to reach current limit in Period 3, and sets the reduced currentlimit I_(LIMIT2) in Period 4. The operation continues with the reducedcurrent limit until the controller detects a period of DCM operationwhere the time to reach current limit is not less than the referencetime t_(LIMMAX). In Period n, the switch is disabled and the inputvoltage is low. The controller has determined that the time to reachcurrent limit in a period of DCM was not less than the reference timet_(LIMMAX). Consequently, the controller sets the current limit to thenominal value I_(LIMIT1) in Period (n+1). The current does not reachcurrent limit in Period (n+1) so the switch is turned off by maximum ontime and the current limit remains at I_(LIMIT1). The power supplyoperates in CCM at low input voltage and current limit I_(LIMIT1) inPeriod (n+2) and Period (n+3).

FIG. 7 shows one embodiment that includes a power switch 736 in anintegrated circuit 700. Power switch 736 is a MOSFET that conductscurrent between a drain terminal 702 and a source terminal 758. Circuitsinternal to the integrated circuit are powered from an internal voltageV_(CC) 705 that is referenced to source terminal 758. For oneembodiment, drain terminal 702 provides internal voltage V_(CC) 705.Internal voltage V_(CC) may be provided from drain terminal 702 or froma different terminal of the integrated circuit by several techniquesthat are known to one skilled in the art.

A feedback terminal 754 receives a feedback signal U_(FB). A modulator752 interprets the feedback signal U_(FB) to set an enable signal 744high or low. An oscillator 756 provides a clock signal 748 and a D_(MAX)signal 746 to determine respectively the length of a switching periodand the maximum on time of the switch 736. Switch 736 may be on whileD_(MAX) 746 is low. Switch 736 is off while D_(MAX) 746 is high. ANDgate 740 sets latch 738 to turn on switch 736 with drive signal 757 atthe beginning of a switching period if the enable signal 744 is high. ORgate 742 resets latch 738 to turn off switch 736 with drive signal 757if switch current I_(D) 706 exceeds the current limit or if signalD_(MAX) 746 goes high.

Switch current I_(D) 706 is sensed as a voltage V_(ID) that is comparedto a current limit voltage V_(ILIMIT) by a comparator 704. Resistor 732with current sources 728 and 730 generates current limit voltageV_(ILIMIT). Current source 730 is switched on and off by p-channeltransistor 724. In one embodiment, current source 730 is one-tenth thevalue of current source 728. Thus, the current limit voltage V_(ILIMIT)increases by 10 percent to make a nominal current limit 10 per centhigher than a reduced current limit when current source 730 is switchedon.

The drive signal 757 that is output by latch 738 is delayed by leadingedge blanking time t_(LEB) delay 734 before being received by AND gate708. AND gate 708 receives the output of current limit comparator 704and the output of leading edge blanking time delay 734 to provide anover current signal 760. Leading edge blanking time t_(LEB) delay 734 islong enough to allow switch 736 to discharge stray capacitance on drainterminal 702. Discharge of stray capacitance at drain terminal 702 canproduce a high drain current I_(D) 706 that temporarily exceeds thecurrent limit, but is unrelated to the output of the power supply. Theleading edge blanking time t_(LEB) delay 734 prevents the over currentsignal 760 from going high during a time t_(LEB) after switch 736 turnson. Over current signal 760 in FIG. 7 corresponds to signal I_(LIM) inFIG. 4 or FIG. 6.

Flip-flop 750 remembers the state of enable signal 744 at the beginningof the switching period. Flip-flop 750 is clocked at the start of everyswitching period by the complement of D_(MAX) signal 746 from inverter720. A change in the state of the clocked enable signal 745 from oneswitching period to the next switching period is detected by XOR gate716.

XOR gate 716 with delay 718 at one input receives the clocked enablesignal 745 to set latch 714 whenever there is a change in the clockedenable signal 745. Delay 718 is long enough to produce an output thatsets latch 714. In one embodiment, delay 718 is ten nanoseconds. Latch714 is set at the beginning of a switching period whenever there hasbeen a change in the state of the clocked enable signal 745 from theprevious switching period.

Latch 726 is allowed to set if enable signal 744 is high at thebeginning of the current switching period. Inverter 722 resets latch 726if enable signal 744 is low at the beginning of the current switchingperiod.

Latch 714 is set to indicate DCM operation in the present switchingperiod. DCM is indicated when the output of latch 714 is high. Latch 726is set to reduce the current limit.

In the embodiment of FIG. 7, the maximum on time signal D_(MAX) 746 isalso the timing reference that is compared to over current signalI_(LIM) 760 to determine whether the current limit will be I_(LIMIT1) orI_(LIMIT2). In the embodiment of FIG. 7, t_(LIMMAX)=t_(DMAX),representing an embodiment where the signals I_(LIMMAX) and D_(MAX) ofFIG. 4 are identical. For another embodiment, however, it is appreciatedthat t_(LIMMAX) does not necessarily have to equal t_(DMAX) inaccordance with the teachings of the present invention, such as theexample illustrated in FIG. 4. Latch 726 will not be set if there is noover current condition detected or the current limit is not reachedduring the time when D_(MAX) 746 is low. Thus, current source 730remains switched on by transistor 724 if the over current condition isnot detected in accordance with the teachings of the present invention.

It is appreciated that although FIG. 7 illustrates an integrated circuit700 for an example of the present invention that employs a switchingregulator that may skip switching cycles of power switch 736 in responseto enable signal 744, other examples of integrated circuits may also becovered in accordance with the teachings of the present invention. Forexample, a pulse width modulated (PWM) regulator circuit may also becovered in accordance with the teachings of the present invention. Forinstance, an example PWM controller deduces the magnitude of the inputvoltage by measuring how long the current takes to go between two valuesand then adjusts the variable current limit in accordance with theteachings of the present invention.

In the foregoing detailed description, the methods and apparatuses ofthe present invention have been described with reference to a specificexemplary embodiment thereof. It will, however, be evident that variousmodifications and changes may be made thereto without departing from thebroader spirit and scope of the present invention. The presentspecification and figures are accordingly to be regarded as illustrativerather than restrictive.

1. A power supply regulator, comprising: a comparator having first andsecond inputs and an output, the first input of the comparator adaptedto sense a current flow through a switch, the second input of thecomparator adapted to sense a variable current limit value; and acontroller coupled to the output of the comparator and to the switch tocontrol switching of the switch to regulate an output of a power supplyin response a feedback signal, wherein the controller disables theswitch if the sensed current flow through the switch is greater than thesensed variable current limit value, wherein the variable current limitvalue is changed by the controller, wherein every time the variablecurrent limit value is changed by the controller, the variable currentlimit value is changed by the controller only in response to an inputline voltage of the power supply.
 2. The power supply regulator of claim1, wherein the variable current limit value is set to a first currentlimit value if there is not an over current condition during a firstswitching cycle that occurs after a skipped switching cycle of theswitch.
 3. The power supply regulator of claim 2, wherein the variablecurrent limit value is set to a second current limit value if there isthe over current condition during the first switching cycle that occursafter the skipped switching cycle of the switch.
 4. The power supplyregulator of claim 3, wherein the first current limit value is greaterthan the second current limit value.
 5. The power supply regulator ofclaim 2, wherein the first current limit value corresponds to a nominalor relatively low input line voltage of the power supply.
 6. The powersupply regulator of claim 3, wherein the second current limit valuecorresponds to a relatively high input line voltage of the power supply.7. The power supply regulator of claim 2, wherein the first switchingcycle that occurs after the skipped switching cycle of the switchcorresponds to a discontinuous conduction mode of operation of the powersupply.
 8. The power supply regulator of claim 2, wherein energy in anenergy transfer element of the power supply goes to zero during theskipped switching cycle of the switch.
 9. The power supply regulator ofclaim 2 wherein the over current condition occurs when the currentthrough the switch exceeds the variable current limit value before amaximum on time of the switch occurs during a switching cycle of theswitch.
 10. The power supply regulator of claim 1 further comprising anintegrated circuit including the controller.
 11. The power supplyregulator of claim 10 wherein the integrated circuit further includesthe switch.